Diode-lasers and arrays of diode-lasers are now almost universally used for optically pumping bulk solid-state lasers and fiber-lasers. In many optical pumping arrangements, the diode-lasers are operated in a continuous-wave (CW) mode. The laser being pumped can be operated in a CW pumped mode or can be operated in a pulsed mode by mode-locking or Q-switching the solid-state laser. It is also possible to operate a diode-laser pumped solid-state laser or fiber-laser in a pulsed mode by operating the diode-laser in a pulsed mode. This, however, is usually only practical for low pump-powers because of power-supply availability.
The overall (electrical to optical) conversion efficiency of a diode-laser can be about 50% or greater. A portion of the residual inefficiency manifests itself as resistive heating.
In a one-dimensional array of diode-lasers, typically referred to as a diode-laser bar, the individual lasers (emitters) are aligned in the slow-axis direction. Generally the more emitters there are, the more total power is emitted, however, the less is the overall brightness of the output. Emitters of the diode-laser bar are formed in epitaxially-grown semiconductor layers on a single crystal semiconductor substrate. The diode-laser bar is typically mounted epitaxial-side down on a heat-sink.
The brightness of an individual diode-laser output in an axis (the fast-axis) perpendicular to the slow-axis is much brighter than that in the slow-axis. In two-dimensional arrays of diode-laser bars diode-laser bars are arranged one above the other in the fast-axis direction. While this provides for more power than would be available with any one of the bars without significant decrease in brightness, the total fast-axis brightness is limited by the distance that is provided between the diode-laser bars. Usually, space is provided for at least a sub-mount to provide some individual cooling and to thermally separate the diode-laser bars for limiting temperature rise due to resistive heating. Sub-mount-separated diode-laser bars in a two-dimensional array thereof are usually separated by about 400 micrometers (μm).
It has been recognized in the prior-art that if a two-dimensional diode-laser array is to be operated in a pulsed-mode, with relatively low power pulses at relatively low pulse-repetition rate (PRF), for example less than about 100 Hz with a duty cycle of about 1%, the resistive heating of the array will be sufficiently low that diode-laser bars in the array can be directly stacked one-on-another, with a cooling member on the “top” and on the “bottom” of the stack, but without any intervening cooling members. Such an arrangement is described in U.S. Pat. No. 5,394,426, and also in U.S. Pre-grant Publication No. 2008/0089371, the disclosures of which are incorporated herein by reference.
Such stacking reduces the spacing (pitch) of the diode-laser bars in the fast axis essentially to the thickness of the substrate on which a bar is grown. This thickness is typically on the order of about 150 μm for a substrate thinned from a commercially available semiconductor wafer. Nevertheless, this provides at least a three fold increase in brightness compared with a stack wherein each bar is individually cooled. It has been observed, however, that, even at a duty cycle of only 1%, the aggregate output spectrum of a directly-bonded stack is significantly broadened and distorted compared with the spectrum of a single diode-laser bar.
In many applications, the shape of the output spectrum of the stack can be as important as the brightness of the output. Accordingly it would be advantageous to control the shape of the output spectrum of a stack of diode-laser bars to maximize the advantage of the higher brightness.